1. Field of the Invention
The present invention relates to a switching amplifier and its modulation process, more particularly, it relates to a non-filter switching amplifier and its modulation process.
2. Description of Prior Art
A conventional pulse-width-modulation (PWM) D-type switching amplifiers with two level output needs output filters, which consisting of the extra inductor and capacitor generally, to reduce the electromagnetic interference (EMI). However, these output filters will increase the cost and size of the system. This has detrimental effects on the portable application.
The dual comparator modulation with three level output is generated in the prior art to overcome this defect. Although the dual comparator modulation does not need any output filters, it can make utmost of all the four work states of the H-bridge. It can send the current to the load only in demand, and the transported current will be completely consumed by the load self without any waste for the back discharge. The opposite power voltage applied on two ends of the load induces the elimination of the current modulation states, which makes it possible to connect the output of the H-bridge with the two ends of the load directly.
The U.S. Pat. No. 3,585,517 suggests a dual comparator modulation with three-level output. FIG. 1 shows one of its simplified schematic drawings. As is seen in FIG. 1, the input signal is sent to input 10, and then into the pulse-width modulator 12 by sum-edge 11. The other two inputs of said pulse-width modulator 12 are two triangular wave signals with a phase difference of 180° from the output 14 and 15 of the triangular wave generator, respectively. The PWM signal from the output 18 and 19 of said pulse-width modulator 12 (namely the input of the H-bridge) is sent to H-bridge 16. Output 20 and 21 on H-bridge 16 are connected with the two ends of load 17. The output pulse signal from output 20 and 21 on H-bridge 16 has the same pulse width to the PWM signal from input 18 and 19 on H-bridge. However, it has higher current driving ability. The output pulse signal from output 20 and 21 on the H-bridge is sent back to sum-edge 11 by the feedback loop.
FIG. 2A gives a typical circuit, which can realize said modulation, to generate the needed three-level output pulse in FIG. 1. Audio input signal 10 is connected with the forward input of two amplifiers of 28 and 29. The output of amplifier 28 is connected with its inverting input by capacitor 31. The inverting input of amplifier 28 is connected with one of the outputs of the H-bridge by resistor 30. Similarly, the output of amplifier 29 is connected with its inverting input by capacitor 33. The inverting input of amplifier 29 is connected with the other output of the H-bridge by resistor 32. One of the inputs of comparator 26 is connected with the output of amplifier 28, and another input is connected with trianglular wave output 15 of triangular wave generator 13. Similarly, one of the inputs of comparator 27 is connected with the output of amplifier 29, and another input is connected with another triangular wave output 14 of triangular wave generator 13. The triangular wave signals from 14 and 15 have the same amplitude, butbut with a phase difference of 180°. The outputs of comparator 26 and 27 are connected with two inputs of the H-bridge, respectively. The two outputs of the H-bridge are connected with two inputs of trumpet 17. The audio input signal and feedback output signal are integrated after being summarized by integral amplifier 28 and 29, and then the difference value between the input and output signals becomes to zero via negative feedback, so as to make the difference output of the H-bridge very close to the audio input. It should take notice that amplifier 28, capacitor 31 and resistor 30 form a low-pass filter essentially, which can provide the loop gain and attenuate the high-frequency component in the feedback output signal simultaneously. Similarly, amplifier 28, capacitor 31 and resistor 30 form another low-pass filter to realize the same function. Pulse-width modulator 34 consists of two comparators of 26 and 27.
FIG. 2B shows the waveforms related to the above dual comparator non-filter modulation. In FIG. 2B, if waveform 23 represents the triangular wave signal from input 14 of PWM 12, waveform 24 indicates the triangular wave signal from input 15 of PWM 12. These two triangular wave signal of 23 and 24 have a phase difference of 180°, but they have the same amplitude. The audio input signal, the number 25 in FIG. 2B, is denoted by the sine wave. The work process of PWM comprises: the area of shadow part, which coincided with audio signal 25 and triangular wave signal 23 and 24, corresponds to one of the forward or reverse pulses from two ends of load 17 (namely the trumpet); the area of the shadow will become larger with the amplitude increase of audio sine-wave signal 25, so the pulse width of the three-level forward or reverse square pulse 26 will also become larger; however, the pulse width of the square pulse will become smaller in FIG. 2B when the amplitude of audio signal 25 achieves the peak value and begins becoming smaller. Feedback loop ensures the waveform of the three-level output pulse, after being low-pass filtered, identical with the input audio sine-wave signal 25. The inner low-pass filtering characteristic of trumpet 17 and the response of mans' ear to audio frequency make this three-level pulse equivalently re-convert to analog audio signal.
FIG. 3A shows another circuit to generate the needed three-level output pulse in FIG. 1, which can realize the said modulation. FIG. 3B gives the waveforms related to this circuit. Similarly, PWM 35 in FIG. 3A consists of two comparators of 26 and 27. A full-differential audio input signal is sent to input 38 and 39, and then into the PWM 35 by integral amplifier 28 and 29 separately. At the same time, single triangular wave signal 40 is another signals sent into PWM 35, which is from output 37 of triangular wave generator 36. In FIG. 3B, waveform 41 and 42 indicate two complement signals of the full-differential audio input. Two output PWM signals from pulse width modulator 35 are sent to H-bridge 16, and finally appear on two ends of load 17. FIG. 3B shows the three-level pulse array on the load, which is consistent with the waveforms obtained in FIG. 2B. If the descriptions for the numbers are the same to that in FIG. 2A, please refer to the descriptions of the numbers in FIG. 2A.
Although the switching amplifier with dual comparator modulation, in the prior art, has more advantages over the traditional switching amplifier with single comparator modulation, the degree of distortion is still unsatisfying due to the non-linear characteristic of the power switching, which limits its advanced applications. Accordingly, the purpose of this invention is to provide a modulation technique with dual comparators, which has the better performance of total harmonic distortion (namely less distortion).